Effect of Water Density on Hydrogen Peroxide Dissociation in Supercritical Water. 1. Reaction Equilibrium

Abstract

Recent experiments showed that the rate of dissociation of H 2 O 2 in supercritical water (SCW) is density dependent and faster than its high-pressure limit rate in the gas phase. These observations suggest that water molecules play a role in this reaction in SCW. We performed density functional theory (DFT) calculations and molecular dynamics simulations to investigate the role of water in H 2 O 2 dissociation. We generated the potential energy surface for H 2 O 2 -water and OH-water complexes by DFT calculations to determine the parameters in an analytical intermolecular potential model, which was subsequently employed in the molecular dynamics simulations. These simulations were performed at different water densities. They provided the structural properties (pair correlation functions) of dilute mixtures of H 2 O 2 and OH in SCW, from which we were able to calculate the number of excess solvent molecules and partial molar volumes for each solute. We used the partial molar volumes for H 2 O 2 and OH to calculate the reaction volume for H 2 O 2 ) 2OH and thereby determined the density dependence of the equilibrium constant for this reaction. The results show that at the reduced temperature of T r ) 1.15 (695 K) the equilibrium constant for H 2 O 2 dissociation is a function of the water density. The mean value of the equilibrium constant changes by less than 5% between 0.25 < F r < 1, but it decreases by an order of magnitude between 1 < F r < 2.75. Knowing the density dependence of the equilibrium constant for this reaction will allow more accurate mechanism-based models of supercritical water oxidation chemistry to be developed. The computational approach applied herein for H 2 O 2 dissociation is general and can be profitably employed to discern the density dependence of the equilibrium constant of any elementary reaction in SCW. There is currently no experimental approach that will provide this information for reactions involving free radicals.

Molecular dynamics study of the hydration of the hydroxyl radical at body temperature

“…8,14,16 Although the basis set used in our study is relatively small, regarding the size of the system, the corresponding binding energy of À25.4 kJ mol À1 compares well with the values calculated at higher levels of theory: À24.5 kJ mol À1 (CCSD(T)/ aug-cc-pVTZ), 8 À23.8 kJ mol À1 (QCISD/6-311++G(2d,2p)), 34 and À22.6 kJ mol À1 (MPW1PW91/aug-cc-pVDZ). 8,14,16 Although the basis set used in our study is relatively small, regarding the size of the system, the corresponding binding energy of À25.4 kJ mol À1 compares well with the values calculated at higher levels of theory: À24.5 kJ mol À1 (CCSD(T)/ aug-cc-pVTZ), 8 À23.8 kJ mol À1 (QCISD/6-311++G(2d,2p)), 34 and À22.6 kJ mol À1 (MPW1PW91/aug-cc-pVDZ).…”

“…2,3 On the other hand, the properties of the hydroxyl radical are commonly exploited by the immune system and prove advantageous in targeted anticancer therapy. [8][9][10][11][12][13][14][15][16][17][18][19] They include ab initio studies of OH in small clusters, mostly consisting of o10 water molecules, density functional theory (DFT) molecular dynamics simulations of OH-(H 2 O) 31 complexes, as well as classical molecular dynamics (MD) and Monte Carlo (MC) simulations. 5,6 Being involved in various chemical and biochemical mechanisms, the hydroxyl radical has become a target of scientific investigations.…”

“…4 The ability of the hydroxyl radical to attack pathogenic microbes and to degrade potentially toxic, nonbiodegradable organic compounds has induced many industrial and technological applications, including advanced oxidation processes, wastewater treatment and biomass processing, in particular. 14,15 Computational studies on the hydroxyl radical in water most often aim at answering two basic questions: (i) the type of water-radical interactions that occur inside the system; and (ii) how the hydroxyl radical affects the solvent structure. Among them, the structural properties of the hydroxyl radical in liquid water seem particularly interesting, since complexation even with a single solvent molecule may significantly affect its reactivity.…”

Acetone hydration in supercritical water: C-NMR13 spectroscopy and Monte Carlo simulation

“…In particular, the hydrogen bond ͑HB͒ formation between solute and waters can play a key role in controlling the reaction mechanism in SCW. [9][10][11][12][13][14][15] It is thus essential to answer the following questions: ''What is the difference in the hydration structure between the ambient and supercritical conditions?'' and ''How is the solute-water HB influenced by the temperature and by the water density?''…”

H2O2‐induced hydrogen bond water networks enhanced on stimulated Raman scattering

“…Spectroscopic characterization of H 2 O 2 in aqueous environments is a recent fascinating research field. Many research studies have been conducted in past years . However, no systematic measurement has been conducted on how H‐bond structure of liquid water affected by H 2 O 2 .…”